The present invention relates to desiccants based on silica gels having an isotherm separation factor within the range of from about 0.07 to about 0.1 and their use in gas-fired cooling and dehumidification equipment.
Desiccant cooling systems, as disclosed more fully in our related U.S. Pat. No. 5,052,188, the disclosure of which is incorporated herein by reference, employ an open cycle to process water vapor between a conditioned space and the environment. The use of thermal energy creates a chemical potential that can be used to produce a cooling effect. If the air can be made dry enough, an evaporative cooler will produce air that is as cold as a conventional electric air conditioner. The overall performance of these systems relies on the quality of the thermal energy input (availability) and the environment as both a cold sink and as a source of chemical potential (unsaturated air). Most solid desiccant cooling cycles consist of a desiccant dehumidifier, a sensible heat exchanger and two evaporative coolers. There are two important modes of operation: 1) The ventilation mode where outdoor air is processed to produce low enthalpy air for the cooled space, and 2) The recirculation mode where air from the cooled space is processed to maintain low enthalpy air conditions in the space.
A schematic of the ventilation mode (a) and a psychrometric representation of the cycle (b) are shown in FIG. 1 of U.S. Pat. No. 5,052,188. Ambient air at (1) is adiabatically dehumidified by the desiccant (DH). The hot, dry air at (2) is cooled by the sensible heat exchanger (HE) to create dry cool air at (3). This air is then adiabatically humidified by the evaporative cooler (EC) to produce cold, nearly saturated air at (4) that enters the building. Simultaneously, an equal amount of building air at (6) is adiabatically humidified to (7). This produces the cold sink for the dry air (3). The air is then heated by the same heat exchanger to (8). This is an attempt to recuperate as much of the heat of sorption as possible from the dehumidification process. The enthalpy of the air must now be increased to (9) by a thermal energy input such as natural gas heating. This air is then passed through the desiccant in order to regenerate it. The warm, humid air that exits the desiccant at (10) is then exhausted to the atmosphere.
A schematic of the recirculation mode (a) and a psychrometric representation of the cycle (b) are shown in FIG. 2 of U.S. Pat. No. 5,052,188. This time ambient air at (1) is adiabatically humidified to (2). This air now becomes the cold sink for the cycle. It is heated by the sensible heat exchanger to (3) and then further heated to (4) by an external thermal input such as natural gas heating. This air is then used to regenerate the desiccant material. The warm, humid air that exits the desiccant at (5) is returned to the environment. Simultaneously, room air at (6) is adiabatically dehumidified by the desiccant to produce warm, dry air at (7). This air is then cooled with the humidified and cooled ambient air to create cool, dry air at (8). The air is then adiabatically humidified to produce the cold, near saturated air at (9) which is returned to the building.
Both of the operational modes previously described operate on a continuous basis. That is, both the dehumidification and the regeneration processes occur at different parts of the cycle simultaneously. In order to accomplish this, the desiccant is deployed into a rotating wheel or drum that continuously cycles the desiccant between the dehumidification and regeneration air streams. The heat exchanger may be of a rotating or static design and the evaporative coolers must be distinctly separate units.
Three important interrelated performance parameters that determine the viability of desiccant cooling systems include:
1. Thermal coefficient of performance (COP).
The thermal COP is the nondimensional ratio of the amount of cooling output that is produced by a given amount of external energy input. For comparative purposes, COP is often quoted at Air Conditioning and Refrigeration Institute (ARI) rating conditions of 95.degree. F. dry bulb and 75.degree. F. wet bulb outdoor temperatures and 80.degree. F. dry bulb and 67.degree. F. wet bulb indoor temperatures. PA1 The EER is a measure of the amount of cooling, in thermal units (BTUs), that is produced by a given amount of parasitic electric energy input for fans, pumps, etc. in electrical units (Watts). PA1 This factor is defined as tons of cooling capacity per 1000 cubic feet per minute of supply air.
2. Parasitic electric energy efficiency ratio (EER).
3. Specific cooling capacity (SCC).
A combination of higher COP values and increased SCC values yields desiccant cooling systems that are more efficient and cost effective than state of the art devices. It has been determined that one of the primary factors affecting these values is the identity of the particular desiccant material employed in the system in terms of its psychrometric performance in the dehumidifier section of the system. For both the dehumidification and regeneration processes, there are two fundamental wavefronts that occur. The first and fastest wave is primarily a thermal front that is most affected by the total amount of thermal heat capacity associated with the dehumidifier. The second and slowest wave is the main concentration wavefront with strong associated thermal effects. Without getting into the details of the physical chemistry involved, it has been postulated that the primary function of an ideal desiccant material in an open cycle desiccant cooling system should be to produce the sharpest possible concentration wavefronts for both the dehumidification and regeneration processes.
Properties of ideal desiccants effecting these characteristics have been determined to include low heat of adsorption, high water adsorption capacity, high diffusivity of water, high chemical and physical stability towards heat, and most importantly, the shape of the desiccant isotherm.
It has been determined that the ideal shape of the desiccant isotherm for use in gas fired cooling/dehumidifier systems has a separation factor ranging from 0.07 to about 0.1 in accordance with the isotherm equation: ##EQU1## wherein X is the normalized loading fraction of water, P is the relative vapor pressure of water and R is the separation factor.
Several adsorption isotherms with different identified separation factors are shown in FIG. 3 of U.S. Pat. No. 5,052,188. Extreme Brunauer Type I isotherm is shown by the curve designated 0.01, whereas linear and less than linear isotherms are shown by the line designated 1.0 and the curve designated 10.0. The ideal isotherm shape is depicted as the area between the curves designated 0.07 and 0.1.
Most of the commercially available desiccant materials have not been developed for the specific purpose of providing space cooling. In most present day applications, the necessity of achieving efficient regeneration as well as deep drying of the air has not been a consideration. The requirement of attaining the very sharp adsorption wavefronts associated with molecular sieves along with the more efficient regeneration characteristics of the desiccant are what makes this application unique commercially. It is not surprising therefore, that none of the commercially available desiccants match the properties needed. The isotherms of various commercial and laboratory-developed desiccants are shown in FIG. 4 of U.S. Pat. No. 5,052,188. The trend is to see linear or nearly linear (Brunauer Type II) isotherms or extreme (Brunauer Type I) isotherms, as compared with the nearly ideal isotherm shape (moderate type I or Langmuir) designated as the "desired shape" for the purposes of this invention. This isotherm has a separation factor (R) of about 0.1.
U.S. Pat. No. 5,052,188 relates to processes for altering crystalline zeolite materials to render them more suitable for use as desiccant materials in gas fired cooling and air conditioning applications and to the achievement of ideal desiccant materials for such applications having an isotherm separation factor within the range of from about 0.07 to about 0.1.
The starting zeolite materials used according to our U.S. Pat. No. 5,052,188, as synthesized or as purchased, yield an extreme type I isotherm when the isothermal properties are evaluated. Therefore such materials must be modified or altered according to the process of our Patent in order to dealuminize the starting material and replace the bulky metallic cations with less bulky hydrogen ions and thereby increase the water absorptive capacity of the zeolite and produce a reprocessed zeolite having an isothermal separation factor within the range of from about 0.07 to about 0.1.
However it is highly desirable to provide a process for directly synthesizing ideal desiccant materials suitable for use in gas-fired cooling and air conditioning systems, i.e., having moderate Type I or Langmuir isotherm shapes, rather than reprocessing conventional zeolites to make them suitable.
Reference is made to U.S. Pat. No. 3,494,874 which discloses the preparation of porous silica gels using metal silicates. The pore sizes range between 20 angstroms and 50 angstroms in radius, or 40 to 100 in diameter. Pores within the diameter range of 20 to 500 angstroms are defined as "mesopores" and pores with diameters equal to or less than 20 angstroms are defined as "micropores" by the International Union of Pure and Applied Chemistry (IUPAC). The gels of this Patent absorb large amounts of water at high relative pressure and therefore the shape of their isotherms appears to be Type IV rather than Type I.
Reference is also made to U.S. Pat. No. 3,313,739 which discloses the preparation of porous silica gels from sodium silicate. The gels of this Patent have pore diameters of 21 angstroms or more and isotherm shapes having a separation factor of about 0.3, as calculated from the examples thereof.
Reference is also made to U.S. Pat. No. 4,894,357 which related to supercritical drying (aerogels). The water absorption data reported in U.S. Pat. No. 4,894,357 indicates the existence of Type IV or Type V isotherm for the materials of this Patent.
Accordingly, it is an object of the present invention to synthesize microporous silica gel materials which achieve a moderate Type I isotherm with a separation factor range of from about 0.07 to about 0.1.
Another object of this invention is to produce microporous silica gel materials which are ideally suited for use as desiccants in gas fired open space air-conditioning and dehumidification systems.